The present invention pertains to a class of device for measuring the intensity of an electric current utility the Faraday effect with an optical fiber which surrounds the current path in a number of windings and which marks an optical path which is exposed to the magnetic field of the current. The optical fiber is guiding a light flux which has a defined polarization state and which undergoes a current-dependent change in its polarization state along this optical path due to the Faraday effect. A polarization measurement unit is then used to record the polarization state of the light after it traverses the optical path and to produce a measurement signal which is characteristic of the current intensity.
A general device of this type is taught, for example, in De-AS No. 28 35 794. In this reference, the optical fiber is wound in N-windings around the electrical conductor in which the electric current to be measured flows. The set of windings is positioned such that the optical path runs, in a very good approximation, in the direction of the components of the magnetic field H generated by the current to be measured. These components surround the conductor in a circular shape. Light of a defined polarization state, in general linearly polarized light, is applied to the fiber, and the polarization state of the light is measured after it traverses the optical path. The magnetic field connected to the current flowing in the conductor causes a Faraday rotation of the polarization plane of the light, the value of which is given by the relation: EQU .theta.=NVI
with the given winding geometry and, in the assumed ideal case, where the fiber is free of disturbing birefringence effects or the disturbing influence of these effects is suppressed, where .theta. is the Faraday rotation, N is the number of windings, I is the current intensity and V is the Verdet constant of the fiber material. The rotation .theta. of the polarization plane of the light exiting at the end of the fiber is measured and analyzed by opto-electronic means and is displayed directly in units of current intensity I. The measurement of the current intensity is thus related to the recording of the rotation .theta. of the polarization plane of the measurement light. The rotation results from the circular birefringence which is generated along the optical path, this birefringence being induced by the Faraday effect and thus being proportional to the current.
For a qualitative elucidation of the measurement problem, let us first refer to the so-called Poincare sphere representation in which the possible linear polarization states of the measurement light are represented by the points on the equator, the two possible purely circular polarization states are represented by the north pole (left-handed circular polarization L) and the south pole (right-handed circular polarization R) and the possible elliptical polarization states of the measurement light are represented by the other points on the surface of a sphere. A linear birefringence .beta. which is present in the optical fiber can be depicted in this representation by a vector which lies in the equatorial plane and the direction of which vector 2.theta..sub.B marks two polarization eigen-states (sometimes referred to as inherent states of natural states) which lie at diametrically opposing points on the equatorial line. If these states are input into the optical fiber, then over its length there would occur no change in the polarization state if only the linear birefringence represented by this vector were present. The length of this vector is a measure of the value of this birefringence. By the same token, a circular birefringence which is present in the optical fiber or is imposed on it in some other way, as well as circular birefringence .alpha..sub.H induced by the Faraday effect, can be represented by a vector which points in the direction of the polar axis of the Poincare sphere. If both linear birefringence and circular birefringence are present, i.e., if overall there is elliptical birefringence, then its polarization eigen-states are given by the direction of the vector .omega. which is the result of the vector sum of the linear birefringence vector .beta. and the circular birefringence vector .alpha..
Viewed along the optical path, in this representation the polarization state of measurement light which is input into the fiber and which has a polarization state other than that which corresponds to the eigen-polarization states undergoes an evolution along a circle on the sphere surface which runs concentrically around the direction marked by the respective birefringence vector .alpha., .beta. or .omega.. This circle originates from the point on the sphere surface represented by the polarization state of the input light and is traversed at an angular frequency for which the "length" of the respective birefringence vector is a measure (see FIG. 4).
Starting with the ideal case where the fiber features no linear birefringence and where the measurement light input into the fiber has the H polarization state (i.e., horizontal polarization), in the Poincare sphere represented a 2.theta. arc of the equator corresponds to a Faraday-effect-induced rotation of the polarization plane which occurs along the optical fiber. The end point C.sub.L of this arc represents the initial polarization state of the measurement light with which the light emerges at the end of the optical fiber which is far from the input point.
In reality, however, there is no optical fiber which is completely free of birefringence. Rather, a real fiber always has more or less intense linear and/or circular or elliptical eigen-birefringence which can result from, for example, deviations in the fiber core from the ideal cross-sectional shape, unilateral elastic stresses in the fiber and circular birefringence caused by twisting of the fiber. The consequence of this is that even when the conductor surrounded by the fiber carries no current, the output polarization state C.sub.L of light which has traversed the fiber does not coincide with the input polarization state H with which it was input into the fiber. Rather, this output light has some elliptical output polarization state which is represented by a point on the Poincare sphere which lies above or below the equator. The curve C(z), which connects point H with point C.sub.L on the Poincare sphere and which describes the development of the polarization state along the fiber from its input point where the light enters with the polarization state H to its end where the exiting light has the "elliptical" polarization state C.sub.L, therefore generally deviates more or less from the equator. In this process this curve C(z) usually also has a complex shape due to unavoidable irregularities in the fiber. In addition, the polarization development along the fiber, and thus the curve C(z) which reflects this on the Poincare sphere, will change in the presence of temperature variations, shocks and, under certain circumstances, aging of the fiber since these effects influence the eigen-birefringence of the fiber. As a consequence, the zero point and the calibration factor of the current measurement device which contains such a fiber as an optical path is subject to fluctuations since, for instance, a temperature-induced shift in the output polarization state C.sub.L of the measurement light cannot be distinguished from a change caused by Faraday rotation of the polarization plane. The variations in the calibration factor are due to the fact that the Faraday rotation has different effects at a point Z on the curve, depending on whether the corresponding polarizatin state C(z) is linearly polarized, i.e., whether it lies on the equator of the Poincare sphere or elliptically polarized (i.e., lies above or below the equator).